Examining the assembly pathways and active microtubule mechanics underlying spindle self-organization

TL;DR

This study investigates the physical mechanisms of spindle self-organization in cell division, revealing distinct pathways to bipolarity and the role of active forces in maintaining proper spindle shape.

Contribution

It introduces a combined experimental and computational approach to analyze spindle shape dynamics and identifies the importance of active pole-pushing forces in bipolar spindle formation.

Findings

Bipolar spindles follow distinct maturation pathways from faulty structures.

Microtubules between poles generate active extensile forces.

Proper force regulation is crucial for bipolarity and preventing multipolar spindles.

Abstract

The bipolar organization of the microtubule-based mitotic spindle is essential for the faithful segregation of chromosomes in cell division. Despite our extensive knowledge of genes and proteins, the physical mechanism of how the ensemble of microtubules can assemble into a proper bipolar shape remains elusive. Here, we study the pathways of spindle self-organization using cell-free Xenopus egg extracts and computer-based automated shape analysis. Our microscopy assay allows us to simultaneously record the growth of hundreds of spindles in the bulk cytoplasm and systematically analyze the shape of each structure over the course of self-organization. We find that spindles that are maturing into a bipolar shape take a route that is distinct from those ending up with faulty structures, such as those of a tripolar shape. Moreover, matured structures are highly stable with little occasions of transformation between different shape phenotypes. Visualizing the movement of microtubules further reveals a fraction of microtubules that assemble between extra poles and push the poles apart, suggesting the presence of active extensile force that prevents pole coalescence. Together, we propose that a proper control over the magnitude and location of the extensile, pole-pushing force is crucial for establishing spindle bipolarity while preventing multipolarity.